The transesterification reaction using methanol and ethanol is well known to the man skilled in the art. It most commonly uses homogeneous catalysts, for example acid catalysts (sulfonic acids, sulfuric acid, etc.), as described notably in U.S. Pat. No. 4,695,411, various metallic compounds, for example metallic salts such as titanium, zinc, magnesium, tin, antimony or lead salts, and these metallic compounds can be used in form of alcoholates, alkyl derivatives or oxides. Preferably, owing to the high reactivity thereof, homogeneous basic catalysts of NaOH, KOH or LiOH type in solution in methanol are more particularly used, or directly alcoholates of these metals, or even certain carbonates such as potassium carbonate and sodium carbonate for example, as mentioned by Freedman B. Et al.: JAOCS 61 No. 10, p. 1638; by Pryde E. H., “Vegetable Oil Fuels”, Proc. Int. Conf., Fargo, N. Dak., 1982, pp. 117-122; and in U.S. Pat. No. 2,383,602.
Transesterification in the presence of methanol is generally carried out in a single catalysis stage in the case of a batch reaction or at least in two catalysis stages in the case of a continuous operation using overflow reactors as described in U.S. Pat. No. 5,354,878.
The ethanolysis methods described are few and much less effective than those using methanol. In fact, with ethanol and with the same alcohol/oil molar ratio than with methanol, it is impossible to obtain naturally separation of the glycerol formed (see notably U.S. Pat. No. 2,383,602).
The solvent power of ethanol being much higher than that of methanol, the consequence is that the glycerin formed during the reaction is made soluble. The conversion to esters is penalized thereby, which does not allow a high conversion in a single reaction stage to be obtained.
A second transesterification stage is thus necessary after removing the glycerin formed from the reaction medium.
Partial ethanol distillation or addition of a certain amount of water or of a third solvent such as a hydrocarbon, n-heptane for example, allows the solubility of the glycerin to be decreased and a sufficient amount thereof to be eliminated, which allows to reach a high conversion in the second catalysis stage. Another option allowing high conversions to be obtained consists in distilling under certain conditions and under reduced pressure the ethyl esters produced during the first transesterification stage.